1. Field of the Invention
The present invention relates to a magnetic disk apparatus for writing and reading data to and from a magnetic recording medium, and to a semiconductor integrated circuit device for use in such an apparatus.
2. Description of the Prior Art
An IC (integrated circuit) designed for use in an FDD (floppy disk drive) (hereafter such an IC will be referred to as an xe2x80x9cFDD ICxe2x80x9d) incorporates analog circuits that handle analog signals, e.g. a read/write circuit for writing and reading data to and from a magnetic disk, as well as digital circuits that handle digital signals, e.g. various control circuits. In most cases, all of these circuits operate on a common supply voltage (typically 5 V). Operating all the circuits on a single supply voltage, however, causes the supply voltage to be contaminated, and thereby made to fluctuate, by the noise resulting from the operation of the digital circuits. This adversely affects the characteristics of the analog circuits. For this reason, in a conventional FDD IC, the circuits incorporated therein are individually provided with a supply voltage terminal to be connected to the supply voltage and a ground terminal to be grounded.
However, in an FDD IC, even if the circuits incorporated therein are individually provided with a supply voltage terminal and a ground terminal, they invariably need to be connected externally to a single supply voltage. This makes it difficult to sufficiently suppress the degradation of the characteristics of the analog circuits, in particular in cases where the analog circuits handle weak signals. Degradation of the characteristics of the analog circuits, more specifically degradation of the characteristics of the read/write circuit, means nothing but degradation of the accuracy with which the FDD apparatus as a whole writes and reads data to and from a magnetic disk.
Moreover, an FDD IC suffers from large current consumption exactly because all of the circuits incorporated therein operate on a common supply voltage.
Moreover, the manufacturing process of an FDD IC includes a high-temperature test that is conducted to check whether the IC operates normally under high temperature and thereby ensure its reliability. This high temperature test requires not only a high-temperature environment, and thus large-scale testing facilities and equipment, but also much time. Accordingly, an FDD IC tends to demand high testing costs, making the costs of the FDD IC as a whole, and thus the costs of the FDD apparatus employing it, unnecessarily high.
Moreover, it is inevitable that the MOS (metal-oxide semiconductor) transistors forming the digital circuits show a certain degree of variation in their performance. If a circuit happens to be composed of MOS transistors of relatively poor performance, the circuit will not offer the desired characteristics. Since there is no way to save such a circuit, it is not possible to achieve a satisfactory yield of an FDD IC. This makes the costs of the FDD IC, and thus the costs of the FDD apparatus employing it, even higher.
An object of the present invention is to provide an inexpensive, low-current-consumption semiconductor integrated circuit device that sufficiently suppresses the degradation of the characteristics of the analog circuits incorporated therein.
Another object of the present invention is to provide an inexpensive, low-current-consumption magnetic disk apparatus that improves the characteristics of the operations associated with the analog circuits incorporated therein.
To achieve the above objects, according to the present invention, in a semiconductor integrated circuit device having analog and digital circuits that operate on a single supply voltage, a regulator circuit is provided whose output voltage is variable and lower than the supply voltage so that the digital circuits operate on the output voltage of this regulator circuit.
According to this circuit configuration, the noise resulting from the operation of the digital circuits is shut off by the regulator, and is thus prevented from contaminating the supply voltage. This helps suppress variation of the voltage that is supplied to the analog circuits for their operation. In addition, the digital circuits operate on a voltage lower than the supply voltage. This helps achieve a reduction in current consumption.
Here, it is to be noted that digital circuits designed for control purposes are expected simply to make correct judgments between a high level and a low level. This means that they do not necessarily have to be operated on a predetermined voltage; that is, they may be operated on a lower voltage, as long as a certain voltage margin is secured to ensure correct judgment between a high level and a low level.
Operating a circuit composed of MOS devices at a voltage lower than its normal operation voltage is equivalent to operating it in a high-temperature environment. Accordingly, in cases where digital circuits composed of MOS devices are used, providing a regulator having an output voltage adjustment terminal so that it varies its output voltage according to the voltage applied to the output voltage adjustment terminal makes it possible to conduct a high-temperature test simply by adjusting the voltage applied to the output voltage adjustment terminal.
In some cases, a circuit that is composed of MOS devices of relatively poor performance and thus does not offer the desired characteristics can be made to offer the desired characteristics by raising its operation voltage. This means that, by raising the output voltage of the regulator, it is possible to save such a circuit. To achieve this, it is preferable that the regulator be, for example, so configured as to vary its output voltage depending on whether a predetermined operation has been performed on a predetermined device provided therein; this makes it possible to fix the output voltage of the regulator at a higher voltage.